Substorm auroral morphology during geomagnetic storms on 19-20 April 2002

In this paper, we report on a study of the auroral activity during three substorm expansion phases that occurred prior to, during and after the magnetic storm main phase on April 20 during which the SYM-H index decreased from -75 to -185 nT. The first substorm actually occurred in the recovery phase of a previous storm on April 19. The IMAGE WIC (the Wideband Imaging Camera) data showed that auroral morphologies of the three events were almost identical. In the first 10 - 20 min after substorm expansion phase onsets, there were significant auroral brightenings and abrupt expansions. In each case, double auroral oval occurred during the later substorm expansion phases and early substorm recovery phases. South-north orientated auroral patches and torches filled in the space between the two ovals. It was found that even with very similar auroral patterns, the three substorm expansion phases may have been very different in terms of auroral electrojet intensity and energy transport from the solar wind into the magnetosphere/ionosphere system. The first substorm event may have been generated by the release of energy stored in the magnetotail, although the trigger was an external IMF northward turning. The second substorm was more likely driven by the intense solar wind dawn-dusk electric field. The third substorm might have been internally triggered by a steady-loading process. Our argument is that the similar auroral morphologies may have been manifestations of different substorm processes, i.e., loading-unloading and/or being directly driven.